Are PHP include paths relative to the file or the calling code?

There are two types of paths: relative, and absolute. Absolute paths start with a /.. (or C:/.. or however that works on Windows) and always unequivocally point at one specific file.

Relative paths (not starting with /) are relative to the include_path variable. That's a variable that contains a bunch of folders, where PHP will look for your relative path. That include_path also includes ., the current directory. The "current" directory depends on which file was the "first" to be executed. So it varies depending on which file starts to include others.

1. You can set your include_path to some specific, unchanging directory, so you always have a fixed path to include to relatively.

2. Better: construct absolute paths:

   include __DIR__ . '/../Login/file.php';
   

   This is always relative to the directory the file is in.

You can't include php files relatively to your webroot that way, cause if you use the slash as first character, the reference will go much deeper than just your document root. So, instead of using your basepath, you could do something like this :

<?php 
   $path = $_SERVER['DOCUMENT_ROOT'];
   $path .= "/yourpath/yourfile.php";
   include_once($path);
?>